In conventional mass flow meters, heat is applied to a sensing tube conducting the fluid to be measured or is directly applied to a fluid and the temperature of the fluid is measured before and after the addition. When the upstream temperature is equal to the unheated stream temperature, mass flow can be measured as inversely proportional to the temperature difference for a constant heat addition. A conventional bridge circuit can be used to obtain an electrical signal versus flow function. In another arrangement, heat is applied to a sensing tube and the temperature of the tube measured before and after the heat addition. The upstream temperature of the fluid is influenced by the heating of the tube and is nearly equal to the heater temperature at zero flow. The mass flow of the fluid is proportional to the temperature differential for a constant heat addition. In a third arrangement, heat is applied to a very small wire, probe or thermistor in the fluid stream and the cooling effect of the fluid stream is measured. Cooling of the element is a function of the mass flow. In still another arrangement, heat is applied uniformly to a tube by resistance heating and the cooling effect of the fluid is measured with thermocouples to determine mass flow.
In yet another arrangement, described in detail in copending application Ser. No. 381,455, filed July 23, 1973, a pair of temperature-sensitive resistance coils are wound around the outer surface of a sensing tube; the coils are heated, and the rate of mass flow of the fluid passing through the tube is measured by a bridge circuit connected to the coils and expressed by voltage output signals. Advantageous features of such an arrangement such as the fast response of the sensor coils or the accuracy of the bridge ratio over wide ambient temperature ranges are, however, offset by disadvantages arising from its attitude-sensitivity which distorts the signals when the tube is tilted. Such sensitivity arises from both internal and external convection currents. In accordance with the invention described and claimed in the above-mentioned application, signal distortion due to external convection currents can be greatly reduced by encasing the sensor coils on the tube in open cell foam. Such insulation is capable of reducing the attitude sensitivity of the metering device by a factor of 100 or more. However, internal convention effects are not reduced. Although small for low-density fluids, e.g. for oxygen at a pressure of 60 psi, in a vertically aligned conduit and for oxygen at a pressure of 1000 psi, the internal convection effect is approximately .+-. 8% of full scale sensor output at no flow.
The present invention reduces the attitude sensitivity of known mass flow meters by eliminating or minimizing the effects of internal as well as of external convection forces. This is accomplished by modifying the shape of the sensing tube. More specifically, the sensing tube is provided with a loop in the form of a single-turn helix which forms a continuous passage for the fluid. In following the arcuate path defined by the loop, the fluid undergoes a reversal of flow direction which has the result of cancelling the effect of convection currents. A temperature gradient developed between a pair of heated, temperature-responsive sensors on the outer surface of the loop, and converted into output signals through an electrical circuit associated with the sensors, is therefore proportional to the true net mass of flow.
Although the primary object of the present invention is to cancel the effects due to internal convection, the loop configuration of the present invention also minimizes external convection currents in mass flow meters with or without sensor insulation.
The advantages of the present invention, both as to its construction and mode of operation, will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures.